The present invention relates to a wheel speed sensor assembly for detecting the rotational speed of e.g. a vehicle wheel.
Typically, as shown in FIG. 10, a wheel speed sensor P is provided opposite to a rotary member B which rotates together with a wheel. The sensor P picks up fluctuations in the magnetic field due to rotation of the rotary member B, converts such fluctuations to electrical signals and outputs the electrical signals. Typically, the rotary member B is a rotor in the form of a pulse ring made of a ferromagnetic material and having teeth (protrusions) 1 formed on the outer periphery thereof, or a pulse ring having its outer periphery magnetized so that N and S poles appear circumferentially alternating with each other.
As disclosed in JP patent publication 9-329612A, the wheel speed sensor P includes two magnetoelectric devices 11 for picking up fluctuations in the magnetic field and producing waveforms that represent fluctuations picked up by the respective magnetoelectric devices 11. The sensor P further includes a signal processing unit for producing a differential waveform from the waveforms produced by the respective magnetoelectric devices 11, and then producing block pulses from the differential waveform based on upper and lower thresholds. The block pulses thus produced are used to calculate the rotation (rotational speed) of the rotary member B (and thus the wheel on which the rotary member B is mounted).
This wheel speed sensor P further includes a high-pass filter circuit which eliminates variations in the output voltage of the magnetoelectric devices 11 and their changes with temperature, and also reduces the influence of eccentricity of the rotary member B, and the influence of external magnetic field noise, thereby improving the detection accuracy (paragraph [0005] of JP patent publication 9-329612A).
Since the wheel speed sensor P is provided near a vehicle wheel, the distance between the magnetoelectric devices 11 of the wheel speed sensor P and the rotary member (rotor) B tends to change due to severe vibration. This may cause changes in the gradient of the differential waveform, thereby producing abnormal block pulses (paragraphs [0011] to [0014] of the above publication).
In order to prevent this problem, as shown in FIGS. 11A and 11B, the differential waveform Vd is continuously and sharply changed, thereby increasing its gradient. FIGS. 11A and 11B show the differential waveforms and corresponding block pulses while the rotor B is rotating in forward and backward directions, respectively. By increasing the gradient of the differential waveform, the rising point of each block pulse Vr stabilizes, which in turn improves the detection accuracy. (See paragraphs [0016] to [0022] of the above publication.)
While this publication proposes to minimize the influence of variations in the distance g between the magnetoelectric devices 11 and the rotor B (FIG. 10), the wheel speed sensor P also tends to vibrate in the rotational directions of the rotor B.
As shown in FIGS. 11A and 11B, block pulses Vr comprise high-level signals and low-level signals. When the differential waveform overshoots the upper threshold Vop, changeover from low-level to high-level signals occurs, and when the rotor B rotates by an angle of α1 (degrees) from this position and the differential waveform undershoots the lower threshold Vrp, changeover from high-level to low-level signals occurs. Then, when the rotor B further rotates by an angle of α2 (degrees), the differential waveform again overshoots the upper threshold Vop, so that changeover from low-level to high-level signals occurs again. The rotational speed of the wheel is calculated from the number of pulses per unit time.
If the wheel speed sensor P vibrates in a direction opposite to the rotational direction of the rotor B by an angle greater than the angle β (degrees) immediately after the differential waveform has undershot the lower threshold, the differential waveform will instantly overshoot the upper threshold and then soon undershoot the lower threshold again when the sensor P vibrates in the opposite direction. Thus, between adjacent pulses, a small pulse is formed. Conversely, if the wheel speed sensor P vibrates in a direction opposite to the rotational direction by an angle β immediately after the differential waveform has overshot the upper threshold, a trough will be formed in a pulse, thus dividing the normal pulse into two abnormal pulses. Thus, every time the sensor P vibrates in the above manner, the number of pulses increases by one compared to the number of normal pulses. This makes accurate detection of the wheel speed difficult. The “normal” pulses herein used refer to pulses each produced when one protrusion (tooth) 1 passes the wheel speed sensor P with the sensor P stationary.
An object of the present invention is to provide a wheel speed sensor assembly which can minimize any harmful influence on the detection accuracy even if the wheel speed sensor vibrates in a direction opposite to the direction in which the rotary member (rotor) B rotates.